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Cipher Home |
In cryptography, a substitution cipher is a method of encryption by which units
of plaintext are replaced with ciphertext according to a regular system; the
"units" may be single letters (the most common), pairs of letters, triplets of
letters, mixtures of the above, and so forth. The receiver deciphers the text by
performing an inverse substitution. Substitution ciphers can be compared with
transposition ciphers. In a transposition cipher, the units of the plaintext are
rearranged in a different and usually quite complex order, but the units
themselves are left unchanged. By contrast, in a substitution cipher, the units
of the plaintext are retained in the same sequence in the ciphertext, but the
units themselves are altered. There are a number of different types of
substitution cipher. If the cipher operates on single letters, it is termed a
simple substitution cipher; a cipher that operates on larger groups of letters
is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the
entire message, whereas a polyalphabetic cipher uses a number of substitutions
at different times in the message, where a unit from the plaintext is mapped to
one of several possibilities in the ciphertext and vice-versa.
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